Air Plan Approval; Oklahoma; Interstate Transport Requirements for the 2010 SO2 NAAQS

Issuing agencies

Abstract

Pursuant to the Federal Clean Air Act (CAA or the Act), the U.S. Environmental Protection Agency (EPA) is approving portions of the State Implementation Plan (SIP) submittal from the State of Oklahoma demonstrating that the State satisfies the interstate transport requirements of the CAA for the 2010 1-hour sulfur dioxide (SO<INF>2</INF>) primary National Ambient Air Quality Standard (NAAQS). This provision requires each State's implementation plan to contain adequate requirements prohibiting the interstate transport of air pollution in amounts that will contribute significantly to nonattainment, or interfere with maintenance, of a NAAQS in any other State.

Full Text

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<title>Federal Register, Volume 91 Issue 194 (Thursday, October 8, 2026)</title>
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[Federal Register Volume 91, Number 194 (Thursday, October 8, 2026)]
[Rules and Regulations]
[Pages 64286-64289]
From the Federal Register Online via the Government Publishing Office [<a href="http://www.gpo.gov">www.gpo.gov</a>]
[FR Doc No: 2026-20640]


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ENVIRONMENTAL PROTECTION AGENCY

40 CFR Part 52

[EPA-R06-OAR-2025-2270; FRL-13190-02-R6]


Air Plan Approval; Oklahoma; Interstate Transport Requirements 
for the 2010 SO2 NAAQS

AGENCY: U.S. Environmental Protection Agency (EPA).

ACTION: Final rule.

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SUMMARY: Pursuant to the Federal Clean Air Act (CAA or the Act), the 
U.S. Environmental Protection Agency (EPA) is approving portions of the 
State Implementation Plan (SIP) submittal from the State of Oklahoma 
demonstrating that the State satisfies the interstate transport 
requirements of the CAA for the 2010 1-hour sulfur dioxide 
(SO<INF>2</INF>) primary National Ambient Air Quality Standard (NAAQS). 
This provision requires each State's implementation plan to contain 
adequate requirements prohibiting the interstate transport of air 
pollution in amounts that will contribute significantly to 
nonattainment, or interfere with maintenance, of a NAAQS in any other 
State.

DATES: This rule is effective on November 9, 2026.

ADDRESSES: The EPA has established a docket for this action under 
Docket ID No. EPA-R06-OAR-2025-2270. All documents in the docket are 
listed on the <a href="https://www.regulations.gov">https://www.regulations.gov</a> website. Although listed in 
the index, some information is not publicly available, e.g., 
Confidential Business Information or other information whose disclosure 
is restricted by statute. Certain other material, such as copyrighted 
material, is not placed on the internet. Publicly available docket 
materials are available electronically through <a href="https://www.regulations.gov">https://www.regulations.gov</a>.

FOR FURTHER INFORMATION CONTACT: Ms. Nevine Salem, EPA Region 6 Office, 
Ozone and Infrastructure SIP Section; telephone number: (214) 665-7222; 
email address: <a href="/cdn-cgi/l/email-protection#fd8e9c919890d393988b949398bd988d9cd39a928b"><span class="__cf_email__" data-cfemail="0d7e6c6168602363687b6463684d687d6c236a627b">[email&#160;protected]</span></a>. Please call or email the contact 
listed above if you need alternative access to material indexed but not 
provided in the docket.

SUPPLEMENTARY INFORMATION: Throughout this document ``we,'' ``us,'' and 
``our'' means the EPA.

I. Background

    Section 110(a)(2)(D)(i)(I) of the CAA requires a State's SIP to 
include provisions prohibiting any source or other type of emission 
activity in the State from emitting any air pollutant in amounts that 
will contribute significantly to nonattainment, or interfere with 
maintenance, of the NAAQS in any other State. EPA has long interpreted 
this language to enact a ``functional prohibition'' on certain 
emissions from upwind States, necessitating the EPA's independent 
assessment whether those emissions will occur or have been adequately 
controlled in the State where they originate. The EPA often refers to 
these requirements as Prong 1 (significant contribution to 
nonattainment of the NAAQS) and Prong 2 (interference with maintenance 
of the NAAQS).
    Additional background for this action is discussed in detail in the 
Federal Register proposal 91 FR 6575 (February 12, 2026). In that 
document we proposed approval of the portions of the infrastructure SIP 
submitted by the State of Oklahoma on May 28, 2021, certifying that its 
SIP meets the transport Prong 1 and 2 infrastructure obligations 
addressing interstate transport for the 2010 1-hour SO<INF>2</INF> 
NAAQS.
    The EPA provided a 30-day public comment period for the February 
12, 2026, proposal, which closed on March 16, 2026 (91 FR 6575, 
February 12, 2026). We received three comment submissions: one opposed, 
one in support, and a letter from the Quapaw Nation through its 
Environmental Office (QNEO). We summarize and respond to these comments 
in section II of this preamble. The full comments are available in the 
docket for this rulemaking.

II. Response to Comments

    Comment: The commenter opposes EPA's proposed approval, asserting 
that the record does not sufficiently demonstrate that Oklahoma's 
emissions will not significantly contribute to nonattainment or 
interfere with maintenance in downwind States. The commenter argues EPA 
should not approve the SIP without more comprehensive analysis, given 
uncertainties in transport modeling and the health significance of 1-
hour SO<INF>2.</INF> They urge supplemental analysis with updated 
inventories, worst-case meteorology, and more comprehensive source 
evaluations.
    Response: EPA considered this comment and continues to find that 
Oklahoma's SIP submission satisfies CAA section 110(a)(2)(D)(i)(I) for 
the 2010 1-hour SO<INF>2</INF> NAAQS. The EPA has consistently applied 
a case-specific weight-of-evidence (WOE) approach focused on near-field 
impacts for SO<INF>2</INF>. As explained in detail in the technical 
supporting document (TSD), because peak 1-hour SO<INF>2</INF> 
concentrations decrease with distance from the source and are more 
localized than ozone or PM<INF>2.5</INF>, EPA identifies an ``urban 
scale'' up to 50 km as appropriate for assessing transport and trends 
and relies on air quality modeling and/or available monitoring, Data 
Requirements Rule (DRR) \1\/designation analyses, and source-specific 
analysis where needed. Consistent with prior SO<INF>2</INF> transport 
evaluations, the proposal applies this 50-km focus and weight-of-
evidence framework to Oklahoma.
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    \1\ 80 FR 51052 (August 21, 2015).

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[[Page 64287]]

    Regarding the data used, the EPA relied on the most recently 
available, complete, and quality-assured data/information at the time 
of the rulemaking. The EPA evaluated 2021-2023 certified design values 
(DVs) at monitors located within 50 km of Oklahoma's borders in 
adjacent States and found they are below the 75 ppb NAAQS. We also 
reviewed statewide and source-specific emissions trends and available 
modeling/DRR analyses. Regarding sources, EPA focused on Oklahoma point 
sources emitting >=100 tpy within 50 km of a State line and found five 
facilities. The EPA then conducted source-specific evaluations of those 
five sources, using emissions, air quality, wind patterns, controls/
retirements, and where available, past modeling analyses. The proposal 
and TSD summarize these evaluations and the basis for concluding that 
Oklahoma does not significantly contribute to nonattainment and does 
not interfere with maintenance in other States.
    The record, as reviewed by EPA, shows no downwind receptors within 
the relevant 50-km ranges recording design values at or above the 
standard, and the facility-level analyses--including cases with prior 
modeling (e.g., Choctaw/Kay Counties) and recent emissions reductions--
do not indicate plausible cross-state contributions to nonattainment or 
maintenance issues in other States for this NAAQS under baseline or 
likely future scenarios.
    In these circumstances, additional photochemical modeling or 
expanded ``worst-case'' air modeling, beyond the WOE approach, is not 
necessary to support the interstate transport determination for this 
NAAQS relative to other States, for the State of Oklahoma. Nor does 
commenter identify with specificity any particular source or emissions 
activity in Oklahoma that, contrary to EPA's analysis, commenter 
believes emits or will emit in violation of the interstate transport 
provision. Finally, EPA notes that this action does not prejudge any 
future determinations; should new certified data or other evidence 
arise, EPA can address it as appropriate under the Act (e.g., future 
transport evaluations or CAA section 126 petitions).
    Comment: A commenter supports the EPA's proposal to approve 
Oklahoma's SIP regarding the 2010 SO<INF>2</INF> NAAQS interstate 
transport provision, citing Oklahoma Department of Environmental 
Quality's (ODEQ's) efforts and noting ``finding no issues within 50 
km'' indicates compliance with the interstate transport requirement, 
and urges EPA to move forward with the approval.
    Response: The EPA acknowledges this comment. As the proposal 
explains, focusing on the 50-km distance reflects the appropriate 
``urban scale'' for SO<INF>2</INF> as an element of EPA's weight-of-
evidence (WOE) approach; but it is not a stand-alone test. EPA's 
evaluation considers multiple lines of evidence, including certified 
design values (DVs) at relevant monitors, emissions and trend data, and 
source-specific analyses for all Oklahoma SO<INF>2</INF> sources 
emitting >=100 tons per year (tpy) within 50 km of State borders, as 
detailed in the technical support document (TSD). Those lines of 
evidence collectively support approval in this case.
    Comment: The Quapaw Nation's Environmental Office (QNEO) requested 
clarification on whether ``certain areas of Indian Country'' that ODEQ 
is asserting regulatory authority over, include areas within the Quapaw 
Reservation, and how ODEQ would meet Tribal engagement/consultation 
requirements for monitoring/modeling or regulatory assumptions inside 
Indian country if relying on the Safe, Accountable, Flexible, Efficient 
Transportation Equity Act of 2005 (``SAFETEA''). QNEO also asked 
whether the EPA intends to provide guidance describing how interstate 
transport SIP rules will apply in Indian country under SAFETEA for this 
and future revisions, including the limits and safeguards necessary to 
prevent infringements on Tribal sovereignty. QNEO seeks greater 
transparency on the technical basis (modeling files, inventories, 
screening analysis) and clarification of any assumptions involving 
sources within or near the Quapaw Reservation. QNEO also requests 
additional information supporting EPA's statement that this action 
``poses no significant risk to human health and the environmental 
conditions in Oklahoma or its Tribal Nations'' for any underlying 
analyses or regional background concentration data used to reach this 
conclusion.
    Response: As explained in section IV, Impact on Areas of Indian 
Country, in the February 12, 2026, proposed action (91 FR 6575) and in 
section III of this final action, the EPA approved the State of 
Oklahoma request under section 10211(a) of SAFETEA on May 12, 2025, to 
administer the State's SIP in certain areas of Indian country including 
within the Quapaw Nation reservation. Neither the May 12, 2025, SAFETEA 
decision, nor the approval of the State's SIP under the CAA, requires 
the State to engage with affected Tribes.
    However, consistent with longstanding Agency policy, the EPA 
encourages Tribes and States to coordinate on environmental matters of 
mutual concern and believes such coordination may be particularly 
useful in this case, where the State is administering a program in 
Indian country. The EPA notes that the Quapaw Nation has CAA Treatment 
as a State (TAS) approval under CAA sections 107(d)(3),\2\ 126,\3\ and 
505(a)(2).\4\ The State is required to comply with affected-state 
status notifications to neighboring States, which includes the Nation.
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    \2\ CAA section 107(d)(3) (Redesignations): Allows a Tribe to 
receive notices from EPA of the need for redesignation of an area or 
portion of their approved areas of Indian country.
    \3\ CAA section 126 (Interstate/Inter-tribal Pollution): Allows 
a Tribe to receive written notifications of new or modified major 
stationary sources that may contribute to air pollution in their 
approved areas of Indian country.
    \4\ CAA section 505(a)(2) (Permit Review): This gives a Tribe 
the opportunity to review and provide comments on Clean Air Act 
Title V permits proposed by other jurisdictions that might affect 
their Indian country.
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    Additionally, the EPA encourages the Quapaw Nation and other 
interested Tribes to coordinate with the State on issues relating to 
implementation of the SIP. EPA appreciates and respects the government-
to-government relationship with the Nation. As such, the EPA will 
continue to engage with and be responsive to the Nation's inquiries and 
concerns about any program administered by EPA or the State. The EPA 
will continue to provide guidance, information, and support to Tribal 
governments and is committed to consultation as appropriate. Specific 
to this action, EPA is making the determination that the Oklahoma SIP 
contains adequate provisions to ensure that the air emissions in the 
State will not significantly contribute to nonattainment or interfere 
with maintenance of the 2010 SO<INF>2</INF> NAAQS in any other State, 
thereby protecting public health. In other words, the Oklahoma SIP 
includes sufficient measures to protect downwind communities in other 
States from SO<INF>2</INF> pollution. This action does not prejudge 
future determinations; should new certified monitoring data or other 
relevant information materially change conditions, EPA may consider and 
act on such information in a future action as appropriate. No changes 
have been made to our final rule because of this comment.
    Regarding technical transparency, the docket contains our TSD, 
which provides the details and references for EPA's determination that 
Oklahoma SO<INF>2</INF> emissions do not significantly impact downwind 
States based on localized impact analyses. The Quapaw Nation

[[Page 64288]]

Reservation is located near the exterior northwestern boundaries of the 
State and therefore within the areas that EPA analyzed. Potential 1-
hour SO<INF>2</INF> emissions impacts are typically found within 50 km 
of sources emitting >=100 tpy. The only SO<INF>2</INF> source in Ottawa 
County, OK, is the Miami Municipal Airport with SO<INF>2</INF> 
emissions of 0.14 tpy reported in 2022. There are no other 
SO<INF>2</INF> sources identified within Ottawa County or the Quapaw 
Nation.
    As the TSD identifies, currently all Oklahoma SO<INF>2</INF> 
monitors show levels well below the NAAQS. In addition, EPA determined 
that current SO<INF>2</INF> DVs in neighboring States in the relevant 
distance ranges are below the NAAQS. The TSD includes the list of the 
five SO<INF>2</INF> sources evaluated that meet EPA's distance and 
emissions quantity criteria, the monitoring and DVs reviewed, and 
source-specific analyses. The docket also identifies all data sources 
(e.g., National Emissions Inventory (NEI), Emissions Inventory System 
(EIS)) used and includes a spreadsheet titled ``Breakdown of 2020 NEI 
SO<INF>2</INF> Emissions from Oklahoma Point Sources Facilities'' for 
an inclusive list. The proposed rule for this action (91 FR 6575; 
February 12, 2026) references SO<INF>2</INF> DRR and available 
designations modeling considered in the analysis. Supporting underlying 
data and other information is available in the docket.

III. Impact on Areas of Indian Country

    As stated in the proposed action (91 FR 6575, February 12, 2026), 
the EPA approved Oklahoma's request to administer all the State's EPA-
approved environmental regulatory programs, including the Oklahoma SIP, 
in the requested areas of Indian country pursuant to section 10211(a) 
of the Safe, Accountable, Flexible, Efficient Transportation Equity Act 
of 2005: A Legacy for Users, Public Law 109-59, 119 Stat. 1144, 1937 
(August 10, 2005) (``SAFETEA'').
    As requested by Oklahoma, the EPA's approval under SAFETEA does not 
include Indian country lands, including rights-of-way running through 
the same, that: (1) qualify as Indian allotments, the Indian titles to 
which have not been extinguished, under 18 U.S.C. 1151(c); (2) are held 
in trust by the United States on behalf of an individual Indian or 
Tribe; or (3) are owned in fee by a Tribe, if the Tribe (a) acquired 
that fee title to such land, or an area that included such land, in 
accordance with a treaty with the United States to which such Tribe was 
a party, and (b) never allotted the land to a member or citizen of the 
Tribe (collectively ``excluded Indian country lands''). In addition, 
the State only sought approval to the extent that such approval is 
necessary for the State to administer a program in light of Oklahoma 
Dept. of Environmental Quality v. EPA, 740 F.3d 185 (D.C. Cir. 2014).
    As explained earlier in this action, the EPA is addressing the 
portions of the Oklahoma SIP that address two of the interstate 
transport requirements for the 2010 1-hour SO<INF>2</INF> NAAQS, as 
these portions meet the requirements in CAA section 110, and 
specifically in CAA section 110(a)(2)(D)(i)(I). Consistent with the 
D.C. Circuit's decision in ODEQ v. EPA and with the EPA's SAFETEA 
approval, these SIP revisions will apply to areas of Indian country as 
follows: (1) pursuant to the SAFETEA approval, the SIP revisions will 
apply to all Indian country in the State of Oklahoma other than the 
excluded Indian country lands as described above; and (2) pursuant to 
the D.C. Circuit's decision in ODEQ v. EPA, the SIP revisions will also 
apply to any Indian allotments or dependent Indian communities that are 
located outside of any Indian reservation over which there has been no 
demonstration of Tribal authority.

IV. Final Action

    The EPA approves the portions of the Oklahoma SIP submitted on May 
28, 2021, that address two of the interstate transport requirements for 
the 2010 1-hour SO<INF>2</INF> NAAQS, as these portions meet the 
requirements in CAA section 110, and specifically in CAA section 
110(a)(2)(D)(i)(I). EPA determines that the Oklahoma SIP contains 
adequate provisions to ensure that the air emissions in the State will 
not significantly contribute to nonattainment or interfere with 
maintenance of the 2010 SO<INF>2</INF> NAAQS in any other State. This 
action is being taken under section 110 of the Act.

V. Statutory and Executive Order Reviews

    Under the Clean Air Act, the Administrator is required to approve 
SIP submission that complies with the provisions of the Act and 
applicable Federal regulations. 42 U.S.C. 7410(k); 40 CFR 52.02(a). 
Thus, in reviewing SIP submissions, the EPA's role is to approve State 
choices, provided that they meet the criteria of the Clean Air Act. 
Accordingly, this action merely approves State law as meeting Federal 
requirements and does not impose additional requirements beyond those 
imposed by State law. For that reason, this action:
    <bullet> Is not a significant regulatory action subject to review 
by the Office of Management and Budget under Executive Orders 12866 (58 
FR 51735, October 4, 1993);
    <bullet> Is not an Executive Order 14192 (90 FR 9065, February 6, 
2025) regulatory action because this action is not significant under 
Executive Order 12866;
    <bullet> Does not impose an information collection burden under the 
provisions of the Paperwork Reduction Act (44 U.S.C. 3501 et seq.);
    <bullet> Is certified as not having a significant economic impact 
on a substantial number of small entities under the Regulatory 
Flexibility Act (5 U.S.C. 601 et seq.);
    <bullet> Does not contain any unfunded mandate or significantly or 
uniquely affect small governments, as described in the Unfunded 
Mandates Reform Act of 1995 (Pub. L. 104-4);
    <bullet> Does not have federalism implications as specified in 
Executive Order 13132 (64 FR 43255, August 10, 1999);
    <bullet> Is not subject to Executive Order 13045 (62 FR 19885, 
April 23, 1997) because it approves a State program;
    <bullet> Is not a significant regulatory action subject to 
Executive Order 13211 (66 FR 28355, May 22, 2001); and
    <bullet> Is not subject to requirements of section 12(d) of the 
National Technology Transfer and Advancement Act of 1995 (15 U.S.C. 272 
note) because application of those requirements would be inconsistent 
with the Clean Air Act.
    This approval of the Oklahoma 2010 SO<INF>2</INF> Interstate 
Transport applies to certain areas of Indian county throughout Oklahoma 
as discussed in section III, and therefore has Tribal Implications as 
specified in E.O. 13175 (65 FR 67249, November 9, 2000). However, this 
action will neither impose substantial direct compliance costs on 
federally recognized Tribal governments, nor preempt Tribal law. This 
action will not impose substantial direct compliance costs on federally 
recognized Tribal governments because no actions will be required of 
Tribal governments. This action will also not preempt Tribal law as no 
Oklahoma Tribe implements a regulatory program under the CAA, and thus 
does not have applicable or related Tribal laws. Consistent with the 
EPA Policy on Consultation with Indian Tribes (December 7, 2023), the 
EPA offered consultation to Tribal governments on February 10, 2026. No 
Tribes engaged in consultation with EPA on this matter.
    This action is subject to the Congressional Review Act, and the EPA 
will submit a rule report to each House of the Congress and to the 
Comptroller General of the United States. This action

[[Page 64289]]

is not a ``major rule'' as defined by 5 U.S.C. 804(2).
    Under section 307(b)(1) of the Clean Air Act, petitions for 
judicial review of this action must be filed in the United States Court 
of Appeals for the appropriate circuit by December 7, 2026. Filing a 
petition for reconsideration by the Administrator of this final rule 
does not affect the finality of this action for the purposes of 
judicial review nor does it extend the time within which a petition for 
judicial review may be filed, and shall not postpone the effectiveness 
of such rule or action. This action may not be challenged later in 
proceedings to enforce its requirements. (See CAA section 307(b)(2).)

List of Subjects in 40 CFR Part 52

    Environmental protection, Air pollution control, Incorporation by 
reference, Interstate transport of pollution, Sulfur oxide.

    Dated: September 30, 2026.
Walter Mason, Regional Administrator, Region 6.

    For the reasons stated in the preamble, the Environmental 
Protection Agency amends 40 CFR part 52 as follows:

PART 52-APPROVAL AND PROMULGATION OF IMPLEMENTATION PLANS

0
1. The authority citation for part 52 continues to read as follows:

    Authority:  42 U.S.C. 7401 et seq.

Subpart LL-Oklahoma

0
2. In Sec.  52.1920(e), the table titled ``EPA-Approved Nonregulatory 
Provisions and Quasi-Regulatory Measures in the Oklahoma SIP'' is 
amended by adding the entry ``Interstate transport for the 2010 
SO<INF>2</INF> NAAQS (contribute to nonattainment or interfere with 
maintenance)'' at the end of the table to read as follows:


Sec.  52.1920   Identification of plan.

* * * * *
    (e) * * *

             EPA-Approved Nonregulatory Provisions and Quasi-Regulatory Measures in the Oklahoma SIP
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                                       Applicable
      Name of SIP provision          geographic or      State submittal   EPA approval date       Explanation
                                   nonattainment area        date
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                                                  * * * * * * *
Interstate transport for the      Statewide..........        05/28/2021  10/8/2026, 91 FR     Approval for
 2010 SO2 NAAQS (contribute to                                            [INSERT FEDERAL      110(a)(2)(D)
 nonattainment or interfere with                                          REGISTER PAGE        (i)(I).
 maintenance).                                                            WHERE THE DOCUMENT  Adequate
                                                                          BEGINS].             provisions
                                                                                               prohibiting
                                                                                               emissions which
                                                                                               will contribute
                                                                                               significantly to
                                                                                               nonattainment in
                                                                                               or interfere with
                                                                                               maintenance of
                                                                                               the 2010 SO2
                                                                                               NAAQS in any
                                                                                               other State.
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[FR Doc. 2026-20640 Filed 10-7-26; 8:45 am]
BILLING CODE 6560-50-P


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